(1) Field of the Invention
The present invention relates to a power supply circuit for supplying electric power to a laser diode which is used for writing and reading digital data on an optical memory medium. Recently, many optical memory devices using an optical memory medium such as an optical disc, optomagnetic disc, optical memory card, and the like, have been developed. The above optical memory devices comprise an optical head, and the optical head comprises a laser diode for writing and reading digital data on the optical memory medium. The laser diode is driven by a small power when reading data, and is driven by a large power when writing data. It is known that a kind of noise called back talk noise will appear in the optical system in the optical memory device mainly due to a resonance phenomenon which occurs in an optical path of a laser beam between the laser diode and the surface of the memory medium. Further, it is known that the SN ratio of the light output of a laser diode is very low when the output level is low. Therefore, it is required to reduce or eliminate the back talk noise and improve the SN ratio in the low output level for stabilizing the reading and writing operations.
(2) Description of the Related Art
FIG. 1 shows a construction of a conventional power supply circuit for supplying an electric power to a laser diode for writing and reading digital data on an optical memory medium which is used in the conventional optical memory device. In FIG. 1, LD denotes a laser diode, PD denotes a photodiode, 1 denotes a second current supply circuit, 1a denotes a current switch circuit, 1b denotes a write current source, 2b denotes a voltage to current converter, 3 denotes a current branch circuit, 3a denotes a current switch circuit, 3b denotes a branch current source, 4 denotes an automatic power control circuit, 5 denotes a high frequency oscillator, 5' denotes a coupling capacitor, 6 denotes a controller, 7a, 7b, and 7c each denote a digital to analog converter, and 10 denotes a current to voltage converter.
In the construction of FIG. 1, the automatic power control circuit 4 outputs a control voltage to supply to the laser diode an electric current which is necessary to emit a predetermined level of an average light power for reading data. The control voltage from the automatic power control circuit 4 is converted to the above current corresponding to the constant average light power for reading data in the voltage to current converter 2b, and the current from the voltage to current converter 2b is constantly supplied to the laser diode LD. When writing data on the optical memory medium, the current switch circuit 1a is made ON under control of a WDT signal from the controller 6, and therefore, the current from the write current source 1b is supplied to the laser diode LD in parallel with the current from the voltage to current converter 2b. The amount of the current from the write current source 1b is equal to the difference between an electric current which is necessary to emit a light power for writing data and the above current from the voltage to current converter 2b, and is preset by the output of the digital to analog converter 7a. The controller 6 supplies a control voltage value V.sub.W -V.sub.R to the digital to analog converter 7a. In addition, a high frequency current is supplied to the laser diode LD from the high frequency oscillator 5 through the coupling capacitor 5'.
The light emitted from the laser diode LD is injected onto the surface of the optical memory medium, and the intensity of the emitted light is monitored by the photodiode PD. A current corresponding to the intensity (power) of the emitted light flows through the photodiode PD. An input terminal of the current to voltage converter 10 and the input terminal of the current branch circuit 3 is connected in parallel to one terminal of the photodiode PD. The controller 6 presets a control voltage (V.sub.W -V.sub.R)' in the voltage to current converter 3b through the digital to analog converter 7c. The amount of the converted output current from the voltage to current converter 3b, corresponds to the difference between the current which flows in the photodiode PD when writing data and the current which flows in the photodiode PD when reading data. The current switch circuit 3a in the current branch circuit 3 is made ON under control of the WDT signal from the controller 6 when writing data. Therefore, a part of the current flowing through the photodiode PD corresponding to the above difference flows in the current branch circuit 3 when writing data, and the average of the current input to the current to voltage converter 10 is equal to the average current component flowing through the photodiode PD corresponding to the current which is supplied from the voltage to current converter 2b to the laser diode LD. The current to voltage converter 10 converts its input into a voltage corresponding to the input, and supplies its output to the automatic power control circuit 4. The automatic power control circuit 4 comprises an integrating error amplifier, further receives a reference voltage V.sub.ref from the controller 6, and compares the above voltage from the current to voltage converter 10 with the reference voltage V.sub.ref to obtains an average error voltage (average difference between the output of the current to voltage converter 10 and the reference voltage V.sub.ref). Thus, the above-mentioned control voltage the amount of which corresponds to the above average error voltage, is suppled to the voltage to current converter 2b, and therefore the current which is supplied from the voltage to current converter 2b to the laser diode LD is controlled so that the average power of a portion of the light which is emitted from the laser diode LD due to the current from the voltage to current converter 2b is maintained at a constant value which is determined by the above reference voltage V.sub.ref.
FIG. 2 shows the operation of the power supply circuit of FIG. 1 as explained above. When the WGT signal is inactive, a low power light which is used for reading data is emitted from the laser diode LD, where the average power in reading data is denoted by P.sub.R. Responding to the active WGT signal, a high power light which is used for writing data is emitted from the laser diode LD, where the average power in writing data is denoted by P.sub.W. The above-mentioned supply of the high frequency current to the laser diode LD in addition to the currents from the voltage to current converter 2b and the second current supply circuit 1, is effective to reduce the aforementioned back talk noise. Further, repeating of On and OFF operations at high frequency is effective to improve the SN ratio in the low output level of the light.
However, the characteristics of laser diodes are generally different between each other, and vary due to aging. FIG. 3 shows examples of the powers of the lights which are emitted from two different laser diodes (or the same laser diode before and after the characteristics change) which have different characteristics from each other in the conventional power supply circuit as shown in FIG. 1. In FIG. 3, a and b each denote a different characteristic curve of the laser diode, and FIG. 3 shows the case of reading data. Due to the above-mentioned automatic power control circuit 4, the average power of the emitted light is controlled to be equal to the constant value P.sub.R regardless of the difference in the characteristics of the laser diodes, however, the amplitude of the high frequency component varies with the characteristic of the laser diode as shown in FIG. 3. For example, in the case as shown by "B'" in FIG. 3, the light which is output from the laser diode cannot be made OFF even in the minimum power level of the light during the high frequency operation for reading data, i.e., the above-mentioned repeating of ON and OFF operations at high frequency are not effectively carried out due to the above change of the characteristics of the laser diode, and therefore the SN ration in the low output level of the light is not effectively improved. Further, when the amplitude of the high frequency modulation is large, the maximum output level of the light emitted from the laser diode exceeds the rated maximum power level of the laser diode as shown in FIG. 2. This excess power output may deteriorate the laser diode.